1. Field of the Invention
The present invention relates to a lamp-lighting control circuit and, in particular, though not exclusively, it relates to controlling the lighting of a lamp.
2. Description of the Related Art
In a projection-type display apparatus (e.g., a liquid crystal projector), light emitted from a light source lamp is illuminated on a light valve (e.g., liquid crystal).
When driven by a signal (e.g., a TV video signal, a video signal from a personal computer, or other signals related to light control as known by one of ordinary skill in the relevant art or equivalents), the light valve modulates the received light so that the modulated light is projected onto a screen through a projection lens.
The projection-type display apparatus includes a controller controlling a variety of elements and operating in accordance with an instruction from an operation unit (e.g., a remote control device or a key-input circuit).
Also, the display apparatus includes an image-signal processing unit and a video signal processing circuit. When the power source of the display apparatus is turned on, the image-signal processing unit performs a variety of initialization processes in accordance with a control signal from the controller and then starts a displaying operation.
A high-voltage discharge lamp (e.g., a metal halide lamp or a halogen lamp) typically serves as the light source lamp.
Light emitted from the lamp is incident on the light valve (e.g., a liquid crystal panel). Video light is obtained by controlling the intensity of the light, which is transmitted through or reflected at the light valve by modulating the liquid crystal panel with a signal from the video signal processing circuit. The video signal is projected onto and displayed on the screen through the projection lens.
When the power source is turned on, each of the electrical circuits can be initiated and the power source lamp can be lighted.
Since a high-voltage discharge lamp generally serves as the power source lamp in a liquid crystal projector, one can apply a high voltage on the lamp at the time of lighting the lamp, thereby causing strong ignition noise at that time.
As a result, the noise sometimes brings an unexpected electrical change to a ground line, a control line, and a power source line and causes errors in (i.e. wrong operations of) peripheral circuits.
In particular, when the temperature around the lamp is high, the lighting of a light source lamp is difficult, resulting in an increase in the number of retries of lighting operations.
The number of occurrences of noise generations increases as the number of retries increases, thereby increasing the possibilities of wrong operations.
Of the wrong operations of the circuits, one of the most influential one occurs when all control terminals are in a LOW state because the Low state of all control terminals brings the image memory in an operation-mode setting state.
Since the operation mode of the image memory is decided in accordance with a data signal on this occasion, if influenced by noise, the display apparatus results in operating in a wrong operation mode set.
Especially, when the image memory is set in a setting-prohibition mode, the display apparatus sometimes does not recover to its normal state without turning on and off its power SW.
When the wrong operation mode is set in the image memory as described above, sometimes an image of a color signal using the image memory is displayed longer than it is wide or in a piecemeal fashion on the screen.
Table 1 shows a function truth table for part of the control signals and setting operations of the image memory. It is understood from Table 1 that mode setting is established when all signals of CS, RAS, CAS, and WE are LOW.
Also, the RAS signal can be High upon data-writing, although when influenced by noise the RAS signal can become LOW, thus bringing the image memory into a mode-register writing state.
TABLE 1Function Truth Table (1/2)Note forStateCSRASCASWEBAADDROperationIdleHXXXXXNOPLHHHXXNOPLHHLBAXILLEGALLHLXBACA, A9ILLEGALLLHHBARALOW ActiveLLLLLOp-CodeMode-RegisterWriteLLHLBAA9NOPActiveLLLHXXAuto-Refresh/(ACT)Self-Refresh 5HXXXXXNOPLHHXXXNOPLHLHBACA, A9ReadLHLLBACA, A9WriteLLHHBARAILLEGALLLHLBAA9Pre-chargeLLLXXXILLEGALReadHXXXXXNOP (Keep LOW(RD)Active Stateafter Burst)LHHHXXNOP (Keep LOWActive Stateafter Burst)LHHLXX1, 2, 4, 8Burst Length:ILLEGAL Full-Page Burst:Burst Stop →LOW ActiveLHLHBACA, A9Suspend Burst &Start New Read 3LHLLBACA, A9Suspend Burst &Start Write 3LLHHBARAILLEGALLLHLBAA9Suspend Burst &Perform Pre-ChargeLLLXXXILLEGALWriteHXXXXXNOP (Keep LOW(WT)Active Stateafter Burst)LHHHXXNOP (Keep LOWActive Stateafter Completionof Burst)LHHLXX1, 2, 4, 8Burst Length:ILLEGAL Full-Page Burst:Burst Stop →LOW ActiveLHLHBACA, A9Suspend Burst &Start New Read 3LHLLBACA, A9Suspend Burst &Start Write 3LLHHBARAILLEGAL
Table 2 shows mode registers. When the influence of lamp-lighting noise causes the RAS signal to become LOW upon data-writing, a value of the data-writing address at that time is wrongly employed as an operation-mode setting data of the image memory.
Since Reserved shown in Table 2 indicates an illegal setting state, once the image memory is wrongly set in the Reserved state, the image memory cannot be returned to the normal state.
Since register-setting especially at the time of operation mode setting is performed when all control signals are LOW, strong noise occurring upon lighting the lamp highly possibly causes all control signals to be LOW.
TABLE 2Mode-Setting Address KeyOperation CodeCAS LatencyBurst TypeBurst LengthA8A7TMA6A5A4CLA3BTA2A1A0BT = 0BT = 100Mode Setting000Reserved0Sequential0001Reserved01Vender001Reserved1Interleave0012Reserved10Use01020104411Only011301188Light Burst Length100Reserved100ReservedReservedA9Length101Reserved101ReservedReserved0Burst110Reserved110ReservedReserved1Single Bit111Reserved111Full PageReserved
Additionally, in order to obtain brighter projection light, the lamp can use a greater amount of power, which is accompanied by an increased level of noise.
Accordingly, making the lamp brighter can lead to more difficulty in fully inhibiting the noise.
Although influence of lamp-lighting can be inhibited by making the housing of the display apparatus larger, this causes the housing to have a greater external size.